September Moon at Perigee


   Our Moon reaches perigee, (closest distance from Earth), for this orbit on Wednesday September 13th. At that time the Moon will be at a distance of 28.99 Earth diameters (369,860 km or 229,820 miles) from the Earth.
   Does our Moon actually go around the Earth as this graphic shows? From our perspective on the Earth the Moon appears to circle around the Earth. However, in reality, the Moon orbits the Sun together with the Earth*

   On the day of the perigee Moon the 23-day old 3rd or last quarter Moon is above the east to southeastern horizon about an hour or so before the Sun rises. The inner planet Venus is very visible shining brightly above the horizon at -3.94 apparent magnitude, and Dwarf Planet Ceres at 7.92 apparent magnitude will not be visible but it is where the graphic indicates it to be.

   *Click here to read my 2006 Scope on the Sky column “The Real Shape of the Moon’s Orbit”. (PDF)

   Read this very informative article about the Earth-Moon system and their orbital motions, written by Joe Hanson. “Do We Orbit the Moon?”

   
   
   

Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.

Mercury at West Elongation

5 June-mercury-at-west-elongation
   On Sunday June 5th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows. Even though it sounds confusing at western elongation for either Mercury or Venus the inner planet will be to the right of the Sun as we view them, meaning that at western elongation an inner planet rises in the east before the Sun rises. And at eastern elongation with the inner planet on the left side of the Sun the inner planet follows the Sun across the sky setting after the Sun sets.
orbital-positions
   From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – out to the left (east) from the Sun to eastern elongation, then reverse and move westward (inferior conjunction) between the Earth and the Sun to western elongation. From there the inner planet moves eastward going behind the Sun (superior conjunction) and eventually reappearing on the eastern side of the Sun for an eastern elongation. Repeat over and over – do not stop!


   Mercury is visible in the morning skies before sunrise as this graphic shows.
   
   
   
   
   

Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Venus-Jupiter Conjunction – Behind the Scenes

June 30

June 30

   Have you been following the motions of the Venus and Jupiter in the evening over the western horizon. The two planets, one an inner planet (Venus) and the other an outer planet (Jupiter), have moved noticeably closer to each other with the closest on Wednesday July 1st at 2 UT. For my time zone, CDT, this was at 10 pm CDT on Tuesday June 30th. At that time they were separated by about 0.5o, about the diameter of the full Moon, or your finger held out at arm’s length. As the days passed the two planets moved apart.

rotating-earth_at-night   So what is happening?
Several things, all involving the motions of Venus, Jupiter, the Earth and the Sun’s apparent motion, and of course the Earth’s rotation. The latter, Earth’s rotation, is the cause of the apparent east to west motion that all celestial objects follow across your skies. Other than being aware of the setting times this sky motion is not a major part of the conjunction between Venus and Jupiter.

   Starting with the Sun we can see over time that the Sun appears to move eastward at a rate of nearly 1o each day, which is the result of the Earth’s orbital motion, aka revolution, of nearly 1o each day. Keep this daily rate in mind because the other planets each move eastward at their own respective daily rate based on distance from the Sun. Venus as an inner planet takes 224.7 days to orbit the Sun so 360o / 224.7 = 1.6o; Earth takes 365.25 days to orbit the Sun so 360o / 365.2 = 0.98o; Jupiter takes 4331 days to orbit the Sun so 360o / 4331 = 0.083o.
Planetary Fact Sheet from NASA.

   What the preceding paragraph boils down to is that Venus and the Sun will catch up with slower moving Jupiter and pass by Jupiter. Venus as a Sun orbiter will move out away from the Sun toward the East and at some point will curve around and head back toward the Sun. Jupiter as an outer planet only moves eastward (excluding when it or any outer planet is in retrograde motion). So the Sun will catch up with Jupiter coming between the Earth and Jupiter, officially on August 26th when Jupiter is at solar conjunction. Venus will also catch with Jupiter but due to Venus being an inner planet it will pass Jupiter twice – east bound and then west bound which is currently the direction Venus is moving. Venus will pass between the Earth and the Sun around the middle of August putting Venus at inferior conjunction.

   This animated graphic shows the sky at the same time and illustrates how the sky shifts toward the west due to Earth revolution as well as the changing positions of Venus and Jupiter. Were you to measure how much the sky shifts daily, by carefully observing the altitude of Regulus, for example, above the horizon and measuring this altitude each evening at the same time from the same location, you would see that Regulus has shifted westward about 1o daily. You may also noticed that Regulus was at its previous days position 4 minutes earlier. So the net effect is that as each day passes the two planets are lower and lower above the horizon, and setting closer and closer to the time of sunset.

   See some pictures of the two planets taken nearly daily since June 19th.

   
   
   
   

Caution: Objects viewed with an optical aid are further than they appear.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Venus at Eastern Elongation

inner-planets-positions   Saturday June 6th Venus reaches the point in its orbit called greatest eastern elongation. As this graphic shows an inner planet, Venus or Mercury, is more or less at a right angle (90o) from the Sun and Earth at eastern elongation. From the surface of the Earth, your backyard, for example, Venus is to the left, or eastern side of the Sun and is setting after the Sun.

   At eastern elongation the angle between Venus and the Sun is at its greatest, which for this elongation Venus will be 45.4o from the Sun. In terms of viewing Venus, this is as about as late as Venus will set, about 4 hours after sunset local time, meaning for some Venus may set close to midnight local time. This animated graphic shows Venus at sunset on June 6th, and then with Venus’s orbit shown, and then with the horizon removed to see where the Sun is relative to Venus.

venus at eastern elongation   From eastern elongation forward Venus will be moving westward toward the Sun, and each day setting closer and closer to the time of sunset. By September Venus will have orbited to inferior conjunction – between the Earth and the Sun. During this part of the inner planet orbit, from eastern elongation to inferior conjunction, the distance from Earth decreases and the apparent size of Venus increases. Venus also goes through phase changes much like our Moon, and so from eastern elongation to inferior conjunction Venus wanes from a small appearing gibbous phase to an increasingly thinner crescent phase.
   
   
   
   

Caution: Objects viewed with an optical aid are further than they appear.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Mars-Mercury Close Conjunction

22apr-bino   Wednesday evening April 22nd at sunset the inner planet Mercury and the outer planet Mars will be within 1-2o from each other as they set over the western horizon. The two should make for a nice view with binoculars as this graphic shows. Actually the two will be close to each other for the next several evenings. However Mars is setting earlier each night while Mercury is moving eastward away from the Sun and Mars. Compare the the relative orbital speeds: Mercury moves around 4o each day while Mars less than 1o. For another comparison look at their relative brightness. Mars has an apparent magnitude of 1.42 compared with the -1.1 for Mercury.

   Viewing the two planets may be a challenge as they are both low above the western horizon at local time for sunset.

   
   
   
   
   
   
Caution: Objects viewed with an optical aid are further than they appear.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

A Hot Seat, or Uranus at Solar Conjunction

   Wednesday, 2 April, the outer planet Uranus is officially on the opposite side of the Sun, or at solar conjunction. At solar conjunction the Earth and the planet on the opposite side pf the Sun are also at what is called heliocentric opposition – that is the two planets are approximately 180 degrees apart using the heliocentric longitudes. Earth: 192o – Uranus: 12o.
   On the opposite side of the Sun an inner planet is said to be at superior conjunction, while an outer planet, Mars outward, is at solar conjunction.

   
   
   
Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Venus at Western Elongation

inner-planets-positions   Today Venus reaches the point in its orbit called greatest western elongation. As this graphic shows the inner planet Venus, or Mercury, is more or less at a right angle (90o) from the Sun and Earth at western elongation. From the surface of the Earth, your backyard, for example, Venus is to the right, or western side of the Sun and is rising before the Sun.

Click on graphic to see it full size.

Click on graphic to see it full size.

   At western elongation Venus, or for that matter Mercury the other inner planet, is as far out from the Sun as we see them and as a result Venus or Mercury will rise at the latest time in this orbit. Today Venus will be 46.6o from the Sun. From western elongation forward Venus or Mercury will be moving eastward toward the Sun and each day rising closer and closer to the time of sunrise. As the planet moves eastward it is moving further away from the Earth toward superior conjunction on the opposite side of the Sun. As the distance between the Earth and Venus, or Mercury, increases combined with the decreasing angle between the planet, the Earth, and the Sun, Venus or Mercury decreases in apparent size and also waxes through gibbous phase shapes but we never see it at a full phase since that is at superior conjunction.

   
   
   
Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.